We have isolated, for the first time, individual efferent nerve terminals from the base of cochlear outer hair cells and propose to investigate several fundamental processes involved in the release of neurotransmitter from these terminals. An hypothesis of efferent function is put forward which is compatible with their purported role in protecting the sensory cells from acoustic trauma. Ionic currents involved in neurotransmission will be identified by the whole-cell patch clamp technique, their activation by electrical and chemical means studied, and their modulation by putative autoreceptors determined. Measurements of plasma membrane capacitance will be used to monitor changes in membrane area as an assay for the fusion events thought to occur during transmitter release. Optical techniques will be applied to investigate the role of cytosolic calcium in release of neurotransmitter and to determine whether adenosine triphosphate, a putative cotransporter, is released from these terminals. Results of all of these investigations will be correlated with the longitudinal position of origin of the nerve terminals since it is known from histochemical data that there is a graded distribution of cholinergic and GABAergic fibers within the cochlea. These data will be used to support or refute standing hypotheses of the role of the medial efferent nerves in the process of hearing.